1. Field of the Invention
The present invention relates to a humidifier device for use in fuel cells and an operating system thereof in entire fuel cell systems. In particular, the present invention relates to a humidifier device for use in fuel cells and a water control device for use in fuel cell systems, which are suitable as a humidifier device and a water control device for humidifying process gases supplied to fuel cells used as onboard power sources or fixed-type small sized electric generators.
2. Description of the Related Arts
Fuel cells are adapted to supply fuels and discharge reaction products continuously and convert chemical energy possessed in fuels directly into electric energy, and have various advantages features such as high energy conversion efficiency, less emission of atmospheric contaminants, low noises and optional choice of scales. The fuel cells are classified in accordance with the kind of electrolytes used into solid polymeric type, phosphate type, alkali type, molten carbonate type and solid oxide type.
Among the fuel cells, solid polymer fuel cells, phosphoric acid fuel cells and alkaline fuel cells generate electromotive force by proton-conducting from an anode to a cathode and it is in common with them that control of a wet state of electrolytes is essential for normal operation of electrolytes.
For example, in a solid polymer fuel cell, a solid polymer electrolyte membrane having a proton conductivity is used as the electrolyte. Specifically, fluoro polymer electrolyte membrane typically represented by a perfluoro sulfonic acid membrane is used generally which is known under the trade name of Nafion (registered trade mark for products manufactured by Du Pont Co.).
The fluoro polymer electrolyte membrane is excellent in oxidation resistance and show good proton conductivity in a wet state but the electric resistance is increased as the water content lowers and can no longer function as the electrolyte membrane. Accordingly, the fluoro polymer electrolyte membrane is usually used in a saturated wet state.
However, since the operation temperature of a solid polymer fuel cell is about 80xc2x0 C., water evaporates from the electrolyte membrane during operation of the fuel cell, and water content in the electrolyte membrane lowers gradually. Further, when proton-conducting from the anode to the cathode, since molecules of water transfer simultaneously, the area for the anode tends to be drying-out particularly. If the situation is left as it is, the electric resistance of the electrolyte membrane increases to generate heat, which lowers the electric power or causes failure.
In view of the above, in the solid polymer fuel cell, process gases supplied to a gas diffusion electrode is generally humidified for properly controlling the wet state of the electrolyte membrane for operating the electrolyte normally.
Further, a phosphoric acid fuel cell uses as an electrolyte, a concentrated aqueous solution of phosphoric acid impregnated in an SiC matrix and the operation temperature is about 200xc2x0 C. Further, an alkaline fuel cell includes a matrix type and a free electrolyte type, in which the matrix type is a fuel cell using, as an electrolyte, an aqueous solution of potassium hydroxide at a concentration of 30 to 45% impregnated into asbestos and operated at a temperature of about 100xc2x0 C.
Also in the phosphoric acid fuel cell and the alkaline fuel cell, it is necessary to properly control the wet state of the electrolyte membrane in order to operate the electrolyte normally and process gases are humidified if the output voltage is lowered or temperature elevation occurs in the fuel cell.
As a humidifier device for humidifying process gases supplied to fuel cells, a humidifier device using steams for humidification and a humidifier device using mists for humidification are known. In the humidifier device using steams, process gases are passed in a humidifying tank (so-called bubbling tank) kept at a high temperature for humidification corresponding to saturated steam pressure, which has a merit capable of stable humidification.
In the humidifier device using mists, finely atomized mists are added to process gases. For example, Japanese Published Unexamined Patent Application Hei 5-54900 discloses a solid polymer fuel cell having a humidifier device for process gases of adding mists finely atomized to 300 xcexcm or less to at least one of fuels or oxidants by using a mist atomizer having a spray nozzle or an ultrasound atomizer apparatus.
In the humidifier device using mists, since sprayed mists are transferred as they are to the electrode on the process gases as a carrier, it provides a merit capable of quantitatively controlling the amount of humidifying water. Further, since the process gases are deprived of latent heat of evaporation when the mists atomize into steams, it has a merit capable of expecting a cooling effect for the process gases.
However, since the humidifier device of humidifying process gases by steams consumes a great amount of energy for generating steams, which causes lowering of energy conversion efficiency of the fuel cell. In addition, since it is necessary to keep the water source at a high temperature to always generate steams during operation of a fuel cell, heating is necessary also for unconsumed water content to further increase the energy loss.
Further, for always generating steams a bubbling bath of a large capacity is required and since a bubbling bath of a large capacity has a large heat capacity, a time constant regarding the change of temperature is increased. Therefore, it requires a long time for elevating the temperature of the bubbling bath and water to a predetermined level to bring about a problem in view of the starting property.
Further, since the time constant relative to the change in temperature is large, the response characteristic is poor, which leads to difficulty in transient control. That is, if a load changes abruptly to require a large amount of process gases and, correspondingly, a great amount of steams, since the amount of humidification cannot be increased abruptly, the water content in the electrolyte membrane becomes insufficient to lower the output.
On the other hand, if an excessive amount of steams is sent to the electrode, so-called flooding in which a gas flow channel is closed by liquid water is caused to lower the output and make the operation state of the fuel cell instable. Particularly, since water is produced by electrode reactions on the cathode, flooding is liable to be caused.
On the contrary, the humidifier device for humidifying process gases by mists as disclosed in Japanese Published Unexamined Patent Application Hei 5-54900 consumes less electric power for the spray nozzle or ultrasound atomizer apparatus and, accordingly, it is efficient. Further, since it is not necessary to wait for the elevation of water temperature for atomizing water, there is no problem for the starting property. Further, since the increase/decrease for the amount of humidification can be controlled irrespective of the water temperature, it is highly responsive.
However, even in a case of a mist humidifier using a spray nozzle or ultrasound atomizer apparatus, it is necessary to change the addition amount of the mists to the process gases continuously corresponding to the change of load for stable operation of the fuel cell. However, Japanese Published Unexamined Patent Application Hei 5-54900 describes only for the control corresponding to the output power but discloses nothing at all for concrete means of continuously changing the addition amount of mists in accordance with change of load.
Further, the spray nozzle or the ultrasound atomizer apparatus has only a small control range for the performance of supplying mists and cannot change the addition amount of mists greatly. For example, in a case of atomizing mists by using a spray nozzle, the addition amount of mists to the process gases can be controlled by a primary air flow rate.
However, a minimum flow rate is present in the primary air flow rate and, if the primary air flow rate is excessively low, the diameter of the mists is excessively large to wet the electrode or drip water from the outlet of the nozzle making it difficult to atomize water.
Further, in a method of obtaining mists by using an ultrasound atomizer apparatus, since the minimum voltage for releasing mists is nearly about 80% of the full voltage, a limit is imposed on great change for the mist supplying amount corresponding to the continuously changing load by merely controlling the input voltage to the ultrasound atomizer apparatus.
Further, when methanol reformate gas is used for the fuel gas, reforming reaction is usually conducted at a high S/C ratio condition (mol number of steams charged/mol number of carbons in charged fuel) in order to reduce carbon monoxide as a catalyst poisoning for the electrode catalyst and, as a result, a great amount of steams are contained in the reformate gases.
However, the S/C ratio is sometimes lowered intentionally, for example, in a case of abruptly increasing the amount of fuel supplied in order to increase the power and, in this case, the amount of steams contained in the reformate gases is decreased. Accordingly, mere determination for the amount of humidification on the side of the anode in proportion with the output power may cause insufficiency or excess for the amount of humidification on the side of the anode, failing to conduct effective water control for the electrolyte.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a humidifier device for use in fuel cells which is used for fuel cells requiring control for the wet state of an electrolyte membrane, capable of humidifying process gases with no excess or insufficiency corresponding to load-change, without lowering the energy conversion efficiency, response performance and starting performance to thereby capable of outstandingly improving the operation stability of the fuel cell.
Further, another object of the present invention is to provide a water control device for use in fuel cell systems which is used for a fuel cell using reformate gases as a fuel and capable of humidifying fuel gases with no excess or insufficiency even if S/C ratio in reforming reaction changes, thereby capable of outstandingly improving the operation stability of the fuel cell.
Additional objects and advantages of the invention will set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, a humidifier device for use in fuel cells according to the present invention has a feature in comprising a mist humidifier unit for atomizing water into process gases supplied to an electrolyte membrane equipped in a fuel cell and a control device for the humidifier unit for intermittently operating the mist humidifier unit in accordance with the operating condition of the fuel cell.
In the humidifier device for use in fuel cells having the constitutions described above according to the present invention, since the mist humidifier unit is intermittently controlled by the control device for the humidifier unit, the amount of the mists added to the process gases can be changed continuously and greatly even when a mist humidifier unit of a performance with narrow control range for the supply of the mists is used.
Accordingly, even if the operating condition of a fuel cell changes depending on the load-change or the like, process gases can be humidified appropriately with no excess or insufficiency and the operation stability of the fuel cell can be improved outstandingly.
Furthermore, since humidification to the process gases are conducted by using mists atomized from the mist humidifier unit, the energy loss is reduced and starting performance and response performance can be improved compared with the steam humidification by using bubbling bath. Further, the consumption power of the mist humidifier unit can be lowered by intermittently operating the mist humidifier unit.
Further, the water control device for use in fuel cell system according to the present invention has a feature in the provision of a mist humidifier unit for adding mists to process gases supplied to an electrolyte membrane equipped in a fuel cell, a water supply unit for supplying water as a reforming material to a fuel reforming device for forming process gases, and the water control device for making a balance between the amount of water added to the process gases by way of the mist humidifier unit and the amount of water supplied by way of the water supply unit to the fuel reforming device in accordance with the operating condition and the control characteristics of the fuel cell.
In the water control device for use in fuel cell systems having the foregoing constitutions according to the present invention, water as the reforming material is at first supplied by way of the water supply unit to the fuel reforming device, and fuel gases incorporated with steams according to the S/C ratio of reforming material are formed in the fuel reforming device. Further, mists from the mist humidifier unit are added to the fuel gases supplied from the fuel reforming device and supplied to the electrolyte membrane equipped in the fuel cell.
In this case, depending on the operating condition of the fuel cell such as load-change and operating condition of the fuel cell system such as change of the S/C ratio in the reforming material, the amount of water supplied to the water supply unit and the amount of water added from the mist humidifier unit to fuel gases are controlled by the water control device such that the total amount of humidification to the process gases takes a predetermined value.
This can humidify the process gases with no excess or insufficiency even if the operating condition of the fuel cell such as load are changed, as well as control characteristics of the fuel cell system such as change of the S/C ratio in reforming material are changed, so that operation stability of the fuel cell system can be improved outstandingly.
According to the present invention, since the mist humidifier unit for atomizing water into process gases supplied to the electrolyte membrane equipped in the fuel cell and the water control device for the humidifier unit for intermittently operating the mist humidifier unit based on the operating condition of the fuel cell are provided, it can provide advantageous effects of reduced energy loss, excellent response performance and improved starting performance.
Further, since the mist humidifier unit is intermittently controlled in accordance with the operating condition of the fuel cell, it can provide an advantageous effect capable of humidifying process gases with no excess or insufficiency even if the load changes greatly.
Further, since the water control device for use in fuel cell system according to the present invention comprises a control device for making a balance between the amount of water supplied from the water supply unit to a reformer system and the amount of water added to process gases from the mist humidifier unit is controlled in accordance with the change of the S/C ratio, when the present invention is applied to a fuel cell of using a steam reforming system for methanol as a fuel supply system, it can provide an advantageous effect capable of humidifying the fuel gases with no excess or insufficiency even if the S/C ratio in the reforming reaction changes abruptly.
Still further, the fuel cell operating system according to the present invention has a feature in the provision of fuel gas supply means for supplying fuel gases to an anode of a fuel cell, oxidizing gas supply means for supplying oxidizing gases (for example, air) to a cathode of the fuel cell, first mist adding means for adding mists to the fuel gases supplied to the anode by the fuel gas supply means, second mist adding means for adding mists to the oxidizing gas supplied to the cathode by the oxidizing gas supply means, first control means for variably controlling an amount of the mists added to the fuel gases by the first mist adding means in accordance with operating condition of the anode and a second control means for variably controlling an amount of the water added to the oxidizing gases by the second mist adding means in accordance with the operating condition of the fuel cell.
As described above, in the humidifier device for use in fuel cells, the water control device for use in fuel cell system and the fuel cell operating system according to the present invention, since the operation stability of the fuel cell system can be improved outstandingly without deteriorating the energy conversion efficiency, starting performance and response performance, for example, if it is applied to a fuel cell system for use in onboard power source, it contributes to the improvement of the drivability and the fuel consumption in automobiles and eventually it serves to great industrial progress.